1. Field
The present disclosure relates generally to energy storage devices, and more particularly to metal-ion battery technology and the like.
2. Background
Owing in part to their relatively high energy densities, light weight, and potential for long lifetimes, metal-ion batteries are used extensively in consumer electronics. In fact, lithium-ion (Li-ion) batteries, for example, have essentially replaced nickel-cadmium and nickel-metal-hydride batteries in many applications. Despite their increasing commercial prevalence, further development of these batteries is needed, particularly for applications in low- or zero-emission hybrid-electrical or fully-electrical vehicles, energy-efficient cargo ships and locomotives, aerospace, and power grids. Such high-power applications will require electrodes with higher specific capacities than those used in currently existing Li-ion batteries.
Sulfur and sulfur-containing compounds have been investigated as a potential source for higher specific capacity electrodes, in addition to offering a number of other advantages, including a high theoretical specific capacity (1672 mAh/g), high energy density, low voltage operation, and relative material abundance. Sulfur's specific capacity is the highest among solid cathode compounds known for rechargeable Li-ion batteries and an order of magnitude greater than currently available commercial cathodes. Its ultra-high specific capacity can enable exceptional gravimetric and volumetric energy densities in rechargeable batteries (e.g., 2600 Wh/kg and 2800 Wh/l, respectively), which is around 4-10 times higher than that of current state of the art Li-ion batteries. Electrochemical reactions in Li/S cells occur at relatively low voltage (e.g., approximately 30-40% lower than that observed in conventional cathodes), allowing greater flexibility in designing electronic components and minimizing safety risks associated with high voltage cathodes. Sulfur is also found abundantly in nature, low cost, and light weight, in addition to having a relatively low toxicity.
For all of these reasons, sulfur-based cathodes are being investigated as a cost-effective, environmentally friendly, performance enhancing component of metal-ion batteries. However, realization of the full potential of sulfur-based cathodes in metal-ion batteries has been hindered by a number of significant challenges, including low electrical conductivity, low ionic conductivity, and the physical instability of conventional sulfur-based cathodes. Sulfur and sulfur-containing compounds are highly electrically insulating. The ionic conductivity of lithium in sulfur and sulfur-compounds is also very small, which typically slows down the overall rate of the electrochemical reactions and leads to low power characteristics in Li/S cells. In addition, sulfur cathodes generate intermediate electrochemical reaction products (polysulfides, such as Li2Sn) that are highly soluble in conventional organic electrolytes. This leads to sulfur cathode dissolution and re-deposition of electrically-insulating precipitates on the anode surface, preventing full reversibility of the electrochemical reaction.
Thus, despite the theoretical advantages of sulfur-based cathodes, practical application in metal-ion batteries is difficult to achieve. Several approaches have been developed to overcome these difficulties, but none have been fully successful in overcoming all of them. For example, some conventional designs have attempted to address the low electrical conductivity by using a conductive carbon additive to form C—S composites, but this does not address the ionic conductivity or cathode instability. Other conventional designs have attempted to address the ionic conductivity by using special electrolytes that cause the sulfur to swell, but this often increases the rate of sulfur dissolution. Still other conventional designs have attempted to improve electrochemical reversibility by eliminating or preventing polysulfide anion precipitation on the anode surface (e.g., via electrolyte additives to dissolve insulating sulfur-containing precipitates), but this does not address the more critical problem of sulfur cathode dissolution, or low electrical and ionic conductivity.
One of the more advanced approaches for sulfur cathode stabilization involves the formation of porous S—C composites by forming a porous carbon matrix and partially filling it with sulfur via melt or solution infiltration. This approach, however, still suffers from low volumetric capacity of the produced composites and still has an unsatisfactorily high cathode dissolution rate.
Accordingly, conventional approaches to address sulfur-based cathode shortcomings have found only limited success. There remains a need for better ways to address the low electrical and ionic conductivity as well as physical instability of sulfur-based cathodes in metal-ion batteries.